The present invention relates to an electrophotographic process and more particularly to a retention type electrophotographic process for forming a plurality of copies of a document from the same and single electrostatic charge image once formed on an electrophotographic photosensitive member.
There have been proposed various processes for forming a plurality of copies of a document. In one known process, a number of copies are duplicated from the same and single electrostatic charge image, which has been once formed on a photosensitive drum, by repeatedly effecting a development with toner and transfer of a toned image onto successive image receiving papers. Usually such a process is referred to as a retention type electrophotographic process. In such a process, in order to obtain a number of copies having good image quality, it is necessary to maintain stably for a long time the charge image once formed on the photoconductive drum. In known processes, since the latent image is composed of an electrostatic charge applied on an upper surface of the photosensitive member, the latent image might be decayed or deteriorated due to undesired escape of electrostatic charge through the toner or undesired injection of electrostatic charge via the image receiving papers from a biased transfer device. Therefore, the electrostatic charge image could not be retained stably on the photosensitive member during the duplication of a plurality of copies, and thus, it is difficult to obtain a duplicated image of good quality over a number of copies of the same document.
In order to avoid such a drawback, it has been known, from a Japanese Patent Application Laid-open Publication No. 54-72053, to use an electrophotographic photosensitive member 1 comprising an electrically conductive substrate 2, a charge retentive layer 3 made of insulating material and applied on the substrate, and a photoconductive layer 4 applied on the charge retentive layer as illustrated in FIG. 1. At first, a primary electrification of one polarity is effected by means of a corona charger 5 and at the same time the member 1 is irradiated uniformly as shown in FIG. 1A. This irradiation may be effected after the primary electrification. During this step, the charges are trapped across the charge retentive layer 3. Then, as illustrated in FIG. 1B, a secondary electrification of an opposite polarity is effected by means of a corona charger 6, while an image of a document to be duplicated is projected upon the photosensitive member 1. Then in an imagewise bright portion L, the charges are trapped across the charge retentive member 3, while in an imagewise dark portion D the charges are trapped across the photoconductive layer 4. Next, the uniform exposure is carried out to remove the charges trapped across the photoconductive layer 4 to form an electrostatic charge image as shown in FIG. 1C. This latent image is formed by only the charges trapped across the insulating charge retentive layer 3 and thus is hardly affected by the development and transfer so that a number of copies can be formed from the same and single latent image. However, in this known process, the edge of the image is liable to become obscure and it is difficult to form a latent image having sufficiently high contrast and resolution. The obscurity at the image edge is assumed to be introduced by the following mechanism. In the secondary electrification together with the imagewise projection shown in FIG. 1B, the positive charge on the free surface of the photoconductive layer 4 and the negative charge trapped in an interface between the photoconductive layer 4 and charge retentive layer 3 are cancelled out by means of carrier pairs generated in the photoconductive layer 4 due to the uniform exposure in FIG. 1C. During this process, as shown in FIG. 2, the carrier pairs 7 produced at the image edge are polarized and held along an irregular electric field 8. Therefore, in the bright portion L, the charge trapped in the interface between the charge retentive layer 3 and photoconductive layer 4 is equivalently spread towards the dark portion D. Further, the known photosensitive member 1 has a relatively thick photoconductive layer 4 and thus, the light image of the document is liable to become obscure.
Another known process for avoiding the obscurity at the image edge has been proposed in an Japanese Patent Application Laid-open Publication No. 55-43566. In this process, use is made of an electrophotographic photosensitive member 11 comprising an electrically conductive substrate 12, a charge retentive layer 13 applied on the substrate, an electrically insulating charge retentive member 14 applied on the layer 13 and a photoconductive layer 15 applied on the layer 14. The successive steps for forming the electrostatic latent image by means of the charges trapped across the insulating charge retentive layer 14 are the same as those shown in FIGS. 1A to 1C. In this process, the charges are trapped on one hand in an interface between the charge retentive layer 13 and insulating charge retentive layer 14 and on the other hand in an interface between the insulating charge retentive layer 14 and photoconductive layer 15, and thus the carrier pairs generated in the photoconductive layer 15 during the uniform exposing step shown in FIG. 1C are prevented from being spread laterally even if the irregular electric field is generated at the image edge. Therefore, the obscurity due to the spread of the charge carriers could be removed to some extent, but the obscurity due to the thick photoconductive layer 15 could not be solved at all.